An attempt was made to determine the validity of our present method for measuring NADH fluorescence over relatively large areas of the cat cortex. It was found that with our present system of calibration, a transient increase in blood pressure (BP) resulted in changes which we would normally interpret as an increase in metabolic activity. When calibration on the basis of a transient rise in BP was used, no changes in "metabolic activity" were present during electrical stimulation or seizure. Since it is more likely that transient changes in BP cause an increase in blood flow than an increase in metabolic activity, we suspect that our present method for determining metabolic activity (changes in NADH fluorescence) is severely contaminated by changes in blood flow.